Effect of Isomerism on the Liquid–Liquid Phase Behavior of

Article Cite This: J. Chem. Eng. Data 2019, 64, 2395−2405

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Effect of Isomerism on the Liquid−Liquid Phase Behavior of Mixtures of 1‑Alkyl-3-methylimidazolium Bis((trifluoromethyl)sulfonyl)amide Ionic Liquids with Heptanol Paulo B. P. Serra,† Xiwen Shao,‡ Arnau Granadero,‡ Marisa A. A. Rocha,‡,# Johannes Kiefer,‡,§,∥ Wolffram Schröer,⊥ Květoslav Růzǐ čka,† Michal Fulem,† and Bernd Rathke*,‡

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Department of Physical Chemistry, University of Chemistry and Technology, Prague, Technická 5, Cz-166 28 Prague 6, Czech Republic ‡ Technische Thermodynamik, Universität Bremen, Badgasteiner Str. 1, D-28359 Bremen, Germany § MAPEX Center for Materials and Processes, Universität Bremen, D-28359 Bremen, Germany ∥ School of Engineering, University of Aberdeen, Fraser Noble Building, Aberdeen AB24 3UE, U.K. ⊥ Institut für Anorganische und Physikalische Chemie, Universität Bremen, Leobener Str. NWII, D-28359 Bremen, Germany ABSTRACT: Systematic investigations on the liquid−liquid phase behavior of binary mixtures of ionic liquids (IL) of the type 1-alkyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)amide (CxmimNTf2, x = 2, 4, 6, 8, and 10) with nheptanol and its branched isomers 3-ethyl-3-pentanol and 2,4dimethyl-3-pentanol have been performed. The phase diagrams were determined at atmospheric pressure in the temperature range (T) of 263−448 K by applying the cloud point method. The phase diagrams are analyzed in terms of well-established numerical concepts based on Ising criticality. The results are discussed, and trends and explanations for the shifts of the range of solubility are given. The systems show partial miscibility with upper critical solution temperatures. Comparing the solutions in branched alcohols with those in the linear isomer, the critical solution points are found to be remarkably shifted toward higher temperatures and lower IL contents. The trend becomes more pronounced for ILs with shorter alkyl side chains where also a change of the shape of the phase diagrams is observed.

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INTRODUCTION Describing structure−property relationships of mixtures of organic solvents is a major topic in chemical and engineering thermodynamics.1 In most cases, a rough understanding of the interactions at the molecular level exists or the solubility data are described by means of different group contribution methods or GE models; for an overview, consider refs 1 and 2. Nevertheless, the description of complicated mixtures of, e.g., ionic liquids and molecular solvents like alcohols remains challenging. Both groups of compounds are forming complex microstructures, which are results of an interplay of different molecular interactions.3 In the case of n-alkyl alcohols, experimental and theoretical work has been focused on this point to unravel both the type and the strength of molecular interactions inside the bulk phase as well as details of the structure and dynamics in the fluids.4−14 The complexity of the systems rises with the size of the molecules, namely, the length, the molar mass, and the tortuosity in the case of isomers. Meanwhile, also interactions and more complex structures in long-chain alcohols and ionic liquids (ILs) can be described at a semiquantitative level,15−17 resulting in a detailed knowledge about H-bonds (for a definition, consider ref 18) and van der © 2019 American Chemical Society

Waals forces and their interplay under different conditions. The temperature dependence of the molecular organization was investigated by different spectroscopic techniques including IR and dielectric spectroscopy.19,20 Different types of local structures and the processes of structuring and reordering were identified.21 In the case of molten organic salts, usually referred to as ionic liquids, the ionic nature has to be considered explicitly. Over the last years, small-angle X-ray scattering22−25 and molecular simulations15,26,27 provided deep insights on the microstructure in these systems. The data showed evidence of a local separation into polar and nonpolar domains within the liquid. The quantitative prediction of the liquid−liquid phase behavior in ionic systems that include long-range interactions is still not possible though. Studies focusing on the description and the unraveling of the molecular interactions in alcohols are manifold.7,13,14 However, all of these findings can only describe the properties of homogeneous mixtures or even dilute Received: December 14, 2018 Accepted: May 16, 2019 Published: May 29, 2019 2395

DOI: 10.1021/acs.jced.8b01203 J. Chem. Eng. Data 2019, 64, 2395−2405

Journal of Chemical & Engineering Data

2396

Value given in the certificate of analysis. bAdditional drying with molecular sieves (3 Å, Merck). cDetermined by Karl Fischer titration. dThe ionic liquids were processed by our standard procedure for degassing and drying prior use, applying a vacuum of p = 2 × 10−5 bar at a temperature of T = 298 K under continuous stirring for at least 48 h. eSigma-Aldrich Chemie GmbH, Steinheim, Germany; Merck KGaA, Darmstadt, Germany; IoLiTecIonic Liquids Technologies GmbH, Heilbronn, Germany.

mol. sievesb vac. dryingd vac. dryingd vac. dryingd vac. dryingd vac. dryingd ≥0.99 ≥0.99 ≥0.99 ≥0.99 ≥0.99 ≥0.98 Mercke IoLiTece IoLiTece IoLiTece IoLiTece IoLiTece 111-70-6 174899-82-2 174899-83-3 382150-50-7 178631-04-4 433337-23-6